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Demographics - How population grows.

I'm not really sure where to put this.

I spend most of my life world-building, pointlessly, and am a member of various groups to assist in that process. In one of these groups, we focus a lot on demographics, and I constantly notice people asking how many people should be in their village or city, and, more often, how fast their population should *GROW*.

It's one of those irritating questions that can never be answered accurately, but I'm ever eager to spout both facts and opinions. So I did some math, then gave up after a couple hours and used an online calculator, and ended up with some statistics on population growth. Since many people here seem to enjoy world-building, and as I've noticed a few mapping threads which focus on the changes in a world/nation over time, I thought I would share a few of these facts and opinions with you:

The crude death rate, the total number of deaths per year per 1000 people, for the whole world is about 8.37 per 1000 per year according to the current CIA World Factbook. The crude birthrate for the entire world is 20.6 live births per 1000 people.

So each year, for every 1000 people on the earth, 20 people are born, and 8 die, increasing the total population by 12. (Actually 20.6 - 8.37 = 12.23, but you can't have a partial person.) That's 1.2 people added to the population each year, per 100, or .0012 per person.

That doesn't sound like a lot does it?

If you have a village with 20 families, (Let's call that Man, wife, no kids yet) that's 40 people. Plus the cranky old guy who lives just outside the bounds of sanity. Just for fun. How fast will your world population grow, if only that village is left alive?

After
1 year - 40.048 people
after 21 years you get a whole new person!

21 years - 41.037 people

20 years later you'll have a population of 42 people:

40 years - 42.002 people

60 years - 43 people

80 years - 44 people So far it seems like the population isn't going anywhere.. 1 person every 20 years isn't much. But this is not a flat rate, it compounds, so the population grows faster (even if it doesn't seem that way for a while.

After 98 years (only 18 years later) you have 45 people (Actually, it's 45.032371604663200050712789175541, in case you were wondering..)

184 years - 50 people.

200 years - 51 people

250 years - 92 people

300 years - 168 people

350 years - 305 people

400 years - 554 people (!)

500 years - 1,825 people

750 years - 36,045 people

1000 years - 711,181 people.

Now, of course, this is all based on averages, of a 1.2% growth rate. It doesn't take into account sudden fluctuations in growth, like wars, or plagues, culture, or quality of healthcare.

Your medieval society might grown at as little as half this rate. (0.6%)

That puts you at 53 people after 50 years, 72 after 100 years, 131 at 200 years, 791 after 500 years, and 15,755 after 1000 years. Quite a vast difference.

How do you decide at what rate your population should grow?

Well, in the very worst of times, your population will die out completely - war, famine, plagues, rampaging wizards...

In the best case scenario, well, the highest birth rate in the world right now is 50 per 1000

The lowest death rate is 1.4 per 1000

That leaves you with a growth rate of 48.6 per 1000, or 4.86%, and a population of 409 after 50 years. 4389 after 100 years, and in 500 years, your 40 villagers would have swelled to 769,798,085,747

A bit unrealistic? Oh yes. But in an advanced society, with no war, medicines to cure anything and ample food (in a large solar system with plenty of room to grow) it's entirely possible.

If you want to play with some numbers on your own, there's a Population Calculator which can help a great deal.

In short - Your population growth depends on your world, and on your people. In general, though, small populations grow slowly, large populations quickly. Low-tech societies grow slowly (or, rather, they die off faster) and advanced societies will grow rapidly without some sort of check in place (like the Two Child Policy imposed by some governments.

Most of the statistics you will find will be worthless, growth-wise, if your world is unique. (No one will die from infection if everyone can magically heal themselves!) The best you can do is to think of all the good and bad in your world and plan consistently.

The biggest thing, as you said, is figuring out what is happening in your world. What factors are killing people and what factors are keeping them alive. Assuming a pre-birth control world there will likely be more births than we see in the modern western world, but with less health care and food, new births are less likely to survive past infancy. With war and such, there are plenty of opportunities for people to die later on.

I think so, for the purposes of fantasy games, people need to consider different factors related to race. Many of the fantasy races have much longer life spans than humans, yet typically lower populations. I think the general assumption is the other races have lower birth rates.

The biggest thing, as you said, is figuring out what is happening in your world. What factors are killing people and what factors are keeping them alive. Assuming a pre-birth control world there will likely be more births than we see in the modern western world, but with less health care and food, new births are less likely to survive past infancy. With war and such, there are plenty of opportunities for people to die later on.

I think so, for the purposes of fantasy games, people need to consider different factors related to race. Many of the fantasy races have much longer life spans than humans, yet typically lower populations. I think the general assumption is the other races have lower birth rates.

You're right, about the lack of birth control, but I think you missed a few things. The availability of birth control also affects people psychologically, and sociologically. If there's no way to prevent or terminate an unwanted pregnancy, a woman (especially a young fertile woman) will be less likely to risk *becoming* pregnant. If there's a *stigma* attached to pregnancy in your society, this is doubly true. And poor/unadvanced health care includes prenatal care, so not only will young children/infants die at a higher rate - there will also be a smaller percentage of live births ... miscarriages, still-births, accidents...

This is why, even without birth control, less advanced societies usually have *lower* birth rates.

If your society views pregnancy without a stigma, this may be less true... for example, a group of survivors hoping to repopulate the earth, or whatever might *strive* for pregnancy... and every woman might pop out a kid every nine months.

As far as different races with long life spans and low populations: It works because they ARE different races. Their bodies, including reproductive systems, may be much different than our own. Maybe the females are only fertile once every 40 years, and hit menopause at 80. That gives them 2 chances to become pregnant, while humans have an opportunity every month. That would make each pregnancy a truly rare and blessed event.

The factors that tend to impact the birth/death rates of most *human* societies the most are poverty, education, war, famine, disease, medical advances and social policy.

But literally *everything* about your world will affect population growth - environment, resources, religion, history, the herbs and animals that exist solely in a fantasy world, magic.. everything.

During much of human history, birth rate was about the same as death. Population did not increased and if it does increase you might have a problem. Unless there is a new technology or something that allows to grow more food on the same space you will end up unable to feed everyone. Less food meens that you might only have have a lower life quality that will reduce birth rate or increase death rate. Or you could have up to 10% of the people die of starvation in 3 years only, like in the 14th centuy.

Another reason why population was not increasing is because of frequent desease and famine. These two where pretty frequent and killed millions of people. Even in the 20th century and today in some poor countries.

War is also a major cause of death. The Taiping rebelion in China is probably the dedliest conflict in human history. It was a civil war and there where other conflicts in the region at the same time !!! if I remember correctly. I guess that's what happends when you loose the mandate of heaven... Not only people die on the battlefield but lands/ villages get burned, many civilians are massacred, and these is still famine and desease because of crops got burned or have rotten in the soil since everybody is busy elsewhere. That is what is going to happend in Game of Thrones...

See also Dragon Magazine #89 "Survival is a group effort" ( http://annarchive.com/files/Drmg089.pdf is a copy that may or may not be legal) for a good discussion of the basic population growth rate equation and a discussion of the two factors (females per female per generation and generation time) might impact how a world works with different races having different generation times.

On the topic of birth control I want to mention that birth control was well known right back to the ancient period, it was just a lot less reliable back then - but it did very much exist. It is a big thing though and when designing a world is worth considering.

The ancient Mediterranean peoples were reported to use an herb known as Silphium that, among other properties, was a highly effective abortifacient. High demand for the herb drove it to extinction in emperor Nero's time, according to reports.

A world with functional magic would have ramifications for population dynamics, especially if the practice of said magic is sufficiently taxing that reproducing female wizards are a near-impossibility. Similarly, the major causes of death in childbirth could be probably reduced to near-zero with a practicing priest in the community (something as simple as D&D's Cure Light Wounds should be sufficient to prevent most of the childbirth-related mortaility, even without more specialized magics aimed at reproduction). Overall, it's not a topic that's discussed much because it doesn't have any significant impact in game terms, but has a HUGE impact in worldbuilding terms. And Americans have traditionally displayed a peculiar aversion to their pimply-faced youths discussing anything to do with sex.

During much of human history, birth rate was about the same as death. Population did not increased and if it does increase you might have a problem. Unless there is a new technology or something that allows to grow more food on the same space you will end up unable to feed everyone. Less food meens that you might only have have a lower life quality that will reduce birth rate or increase death rate. Or you could have up to 10% of the people die of starvation in 3 years only, like in the 14th centuy.

Another reason why population was not increasing is because of frequent desease and famine. These two where pretty frequent and killed millions of people. Even in the 20th century and today in some poor countries.

War is also a major cause of death. The Taiping rebelion in China is probably the dedliest conflict in human history. It was a civil war and there where other conflicts in the region at the same time !!! if I remember correctly. I guess that's what happends when you loose the mandate of heaven... Not only people die on the battlefield but lands/ villages get burned, many civilians are massacred, and these is still famine and desease because of crops got burned or have rotten in the soil since everybody is busy elsewhere. That is what is going to happend in Game of Thrones...

Which is why population growth isn't stable. If you follow the *projected* growth at our current average rate, we *should* already have over 700 billion people on Earth, instead of 7 billion. Thank goodness earlier generations were more in control! (With or without birth control ) And wars have decimated entire populations for centuries! While I don't believe war is ever a good thing (in the real world) it does tend to keep the population from spreading too rampantly.

On the population per food thing, though.. you do have to be careful not to be too rigid with it. Populations DID grow, they simply grew extremely slowly. Babies are born. It just happens. And not all societies saw population exceeding food sources as a bad thing. Consider more recent/well known *tribal* societies' behaviors and it's easier to understand how things may or may not have worked.... Many tribal people would marry their children off to other tribes. Part of the decision to begin that practice was surely a practical one. And when food sources get low, groups of people would go out looking for new food sources, and either split from, or move the tribe to the new source.

And yet, the birth rates for those societies (the ones that do still exist in modern times) is often higher than the birth rates in most countries, (then again, so is the crude death rate), so you do still have a narrower growth rate. It's absurdly hard to measure, though. Different organizations try and come up with different results... Consider these Birth Rate lists.

I digress... Also I would laugh so hard if *everyone* died due to crop losses in GoT!

Originally Posted by waldronate

See also Dragon Magazine #89 "Survival is a group effort" ( http://annarchive.com/files/Drmg089.pdf is a copy that may or may not be legal) for a good discussion of the basic population growth rate equation and a discussion of the two factors (females per female per generation and generation time) might impact how a world works with different races having different generation times.

What a great resource!! Thank you! Most people tend to overdo their gender/population ratios in one way or another when writing about human races, but it's possible to have VERY dramatic differences with other races!

And life expectancy is a HUGE factor!

Originally Posted by Larb

On the topic of birth control I want to mention that birth control was well known right back to the ancient period, it was just a lot less reliable back then - but it did very much exist. It is a big thing though and when designing a world is worth considering.

A very valid point. Related strongly to world-building, as well, since birth control methods/knowledge might exist (or not exist) differently on certain worlds. (Perhaps the only way to end a pregnancy on planet X is by visiting a witch doctor who charges 10 years of your life! I wonder how that would affect your population?!)

Originally Posted by waldronate

The ancient Mediterranean peoples were reported to use an herb known as Silphium that, among other properties, was a highly effective abortifacient. High demand for the herb drove it to extinction in emperor Nero's time, according to reports.

A world with functional magic would have ramifications for population dynamics, especially if the practice of said magic is sufficiently taxing that reproducing female wizards are a near-impossibility. Similarly, the major causes of death in childbirth could be probably reduced to near-zero with a practicing priest in the community (something as simple as D&D's Cure Light Wounds should be sufficient to prevent most of the childbirth-related mortaility, even without more specialized magics aimed at reproduction). Overall, it's not a topic that's discussed much because it doesn't have any significant impact in game terms, but has a HUGE impact in worldbuilding terms. And Americans have traditionally displayed a peculiar aversion to their pimply-faced youths discussing anything to do with sex.

That would certainly slow childbirth related mortality!! But without proper *knowledge* of the workings of a human body, they may still miss many of the problems that lead to miscarriages, or pre-term births! And would a spell that cures wounds work on a not-yet fully developed heart or brain or lungs?

And yeah, I was hesitant to post the topic because it's usually taboo.... but it *does* affect population growth, which has a huge impact on world building ... (doubly so in regards to books as opposed to game development.)

By OzzyKP
I think so, for the purposes of fantasy games, people need to consider different factors related to race. Many of the fantasy races have much longer life spans than humans, yet typically lower populations. I think the general assumption is the other races have lower birth rates.

I only can add a biologist point of view here. What is generally observed (without considering external events such as diseases, war, access to a good healthcare, etc.) is a kind of rule like : "the more an organism has a big lifespan, the more it tends to have a long childhood status, big needs and a low fertility/birth rate".

Human is a kind of a "super predator" in our world, with a long lifespan, a long childhood and a low birthrate. On the contrary, a lot of insects or small organisms have a short lifespan but multiplies quickly. And, yes, there's often (but not necessarily) a correlation between size and lifespan.

And on the reproductive magic front, a sufficiently advanced magic would most likely allow for selection of healthy offspring characteristics, number of offspring per birth, and so on. It would also potentially allow for the development of external non-human uterine replicators to allow for potentially unlimited numbers of offspring for a sufficiently adept or rich individual. Even application of magic to the two basic limiters in a population (food and hygiene) would make a huge difference in how the world develops (the longer unrestrained breeding can occur, the longer the population rides that J-curve).

In my opinion, the critical long-term drivers for the success of a culture are:
1) an increasing population (the number of geniuses per capita seems to be very roughly constant),
2) sufficient surplus in the culture to allow for specialization (if everyone is a subsistence farmer, you're not going to develop too much knowledge on any one subject),
3) a way and will to broadcast persistent information (writing + printing press to let everyone know about that neat stuff that your specialists have discovered).